Many proteins that control cell division are involved in cancer development. The phosphorylation state of the Retinoblastoma protein (Rb) regulates the ability of this protein to control cell proliferation. Phosphorylation of Rb stimulates cell cycle progression, whereas dephosphorylation of Rb inhibits progression through the cell cycle. Alterations in the Rb pathway that lead to excessive phosphorylation of Rb have been observed in almost all types of cancer. In this proposal we present evidence that the phosphorylation state of Rb plays an important role in apoptotic cell death. In fact, it may be that elevated phosphorylation of Rb observed in cancer cells may not only cause a proliferation advantage but resistance to apoptosis. In this work we will test the hypothesis that dephosphorylation of certain amino acids of Rb leads to the caspase-mediated c-terminal cleavage of Rb, which triggers degradation of Rb and apoptosis. We will determine the functional connection between Rb phosphorylation state and Rb cleavage in normal and cancer cells. The requirement for Rb cleavage in the mechanism of action of several apoptosis-inducing agents will be examined. Finally we will investigate whether the transcription factor E2F1 plays a critical role in apoptosis induced by the cleavage of Rb. The proposed studies will elucidate the importance of Rb phosphorylation, Rb cleavage and E2F1 in apoptosis in cancer. PUBLIC HEALTH RELEVANCE: In cancer, cells divide and proliferate in an uncontrolled manner. Thus, the investigation of the molecules that control cell division may lead to information needed to understand cancer cell development and growth. This project is designed to study the function of a protein called Retinoblastoma which is implicated in many types of cancer.